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96 Cards in this Set
- Front
- Back
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A conductor's resistance is:
a. directly proportional to its cross-sectional area b. directly proportional to its diameter c. inversely proportional to its cross-sectional area d. not affected by the cross-sectional area e. measure un micro-Newtons |
c. inversely proportional to its cross-sectional area |
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A potential of 100 volts is applied to a resistor of 60 ohms. Determine the current flow in amps. |
1.667
volts = amps * ohms therefore, amps = volts / ohms
amps = 100 volts / 60 ohms amps = 1.667 |
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A basic law of electricity is unlike charge attract and like charges:
a. repel b. attract c. are neutralized d. reduce power output e. hold each other together
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a. repel |
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4 resistors are connected in parallel. 2 ohms, 18 ohms, 40 ohms and 86 ohms. What is the total resistance in ohms. |
1.69ohms
1/2 ohms + 1/18 ohms + 1/40 ohms + 1/86 ohms = 0.592
1/0.592 = 1.69 ohms
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Under normal circumstances, the heat produced by the electrical wires of a circuit is very small due to:
a. large diameter of conductor b. low resistance in the conductor c. long length of the conductor d. insulation around the conductor e. small diameter of the conductor |
b. low resistance in the conductor |
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What is the total resistance of three resistors in parallel. 10-25-50 ohms.
a. 0.16 ohms b. 6.25 ohms c. 85 ohms d. 62.5 ohms |
b. 6.25 ohms
1/10 ohms + 1/25 ohms + 1/50 ohms = 0.16
1/0.16 = 6.25 ohms |
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Atoms that have greater than 3 electrons in the outer shell are:
a) positively charged |
e) not good conductors |
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An electric motor develops 280kW power output. The motor efficiency is 60%. If the supply voltage is 2.4 kV, determine the draw of the motor in amperes. |
194.444 amps
280,000W / 0.6 = 466,667W
P[watts]= I[amps] * E[volts] therefore, I = P/E I = 466,667W / 2400V I= 194.444 amps |
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A material that conducts electrical current is called
a) a volt |
b) a conductor |
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Electrical power can be determined by the formula
a) P = IE |
a) P = IE |
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When a number of electrons pass a given point over a specific time period the energy may be referred to as
a) an ampere |
a) an ampere |
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An insulator is material that will not _________________ an electrical current.
a) reduce |
e) conduct |
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The basic unit of mechanical or electric power is the
a) volt |
b) watt |
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The basic unit of electrical power is the watt, which is the result of
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c) volts times amperes |
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The opposition a device or material offers to the flow of electric current, is known as:
a) a resistance |
a) a resistance |
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Iron and steel may be magnetized by rubbing the metal with
a) stainless steel |
d) a magnet |
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When current flows in a conductor
a) A.C. voltage is generated |
e) a circular magnetic field is set up around the conductor |
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Current flow direction and intensity will
a) affect magnetic field direction and intensity |
a) affect magnetic field direction and intensity |
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The minimum voltage generated by a single loop DC generator occurs when the conductor is cutting the magnetic lines of forces at an angle of
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d) 180 degrees |
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When magnetic flux lines travel and meet in opposing directions the result can be
a) generator action |
d) motor action |
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When using the Left Hand Rule, the index finger represents
a) generator action flux direction |
d) motor action flux direction |
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When a conductor is moved to cut magnetic flux lines at an angle of 90 degrees
a) maximum resistance to current flow is attained |
d) voltage generated will be maximum |
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A current carrying conductor being moved by a
a) magnetic flux |
e) motor action |
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The total number of lines of force per square metre in a magnetic field is
a) known as the EMF |
c) called the magnetic flux |
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The maximum voltage generated by a single loop DC generator occurs when the conductor is cutting the magnetic lines of force at an angle of
a) zero degrees |
c) 90 degrees |
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A conductor supplied with current while in a magnetic field will
a) tend to move at right angles to the flux lines |
a) tend to move at right angles to the flux lines |
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The property of resistance to conducting magnetic lines of force is known as
a) resistance |
b) reluctance |
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The direction of the ______________ due to motor action may be determined using the Left Hand Rule.
a) force |
a) force |
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A property of a magnet is that it will always point in the north and south direction, when
a) submerged in oil |
e) freely suspended in the air |
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If two unlike poles are placed adjacent to each other there is
a) a force of repulsion between the magnets |
d) a force of attraction between the magnets |
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Faraday discovered
a) the Right Hand Rule for conductors |
b) that a conductor will have a voltage induced if it cuts magnetic flux |
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Magnetic field direction and intensity depend on the
a) temperature of the EMF |
d) direction and intensity of current flow |
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If a conductor is held stationary and a magnetic field is moved across it, it forms what is known as
a) motor action |
c) generator action |
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The black demand pointer of a power meter
a) moves the red pointer |
e) must be manually reset |
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Utility companies have a surcharge to their customers based on
a) the maximum voltage used |
e) peak demand |
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The most common measurement made with a meter is
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d) voltage |
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A clamp-on ammeter measures current flow by using
a) differential voltage |
c) a basic transformer principle |
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Clamp-on ammeter readings may be inaccurate if
a) jaws are dirty or misaligned |
a) jaws are dirty or misaligned |
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Batteries
a) do not pose any immediate electrical hazards |
d) produce a steady DC voltage |
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A kilowatt hour meter has a small motor which
a) turns at a constant speed |
c) has a speed proportional to power consumption |
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A voltmeter is connected to a circuit
a) in series |
d) in parallel |
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Ammeters are connected with the negative or black lead connected to the
a) positive side of the circuit |
c) negative side of the circuit |
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When reading the dials of a power meter, read
a) them in the sequence shown by the letters under each |
d) them from right to left |
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When using an electrical meter to check that a circuit is safe to work on, it is important to verify the proper scale, current, or both that is being used because
a) it may appear that no voltage or current is present and therefore safe |
a) it may appear that no voltage or current is present and therefore safe |
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Motor designation like open, drip proof, explosion proof refers to _____
a. frame number |
b. enclosure type |
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In order to produce the strength of magnetic flux required in larger DC machines
a) electromagnets are used |
a) electromagnets are used |
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In a series wound DC generator:
a) brushes do not change AC to DC |
c) load current passes through the field windings |
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The field strength of an alternator can be adjusted by varying the amplitude of the DC power supplied to the
a) rotating field |
a) rotating field |
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The rotating field of an alternator is obtained by exciting the windings on the rotor with
a) AC power |
d) DC power |
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The ________________________ of a DC machine is built up of laminated steel plates mounted on the shaft. The coils are connected to a commutator mounted on one end of the
a) armature |
a) armature |
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The stator of an alternator is built of ________________ mounted in a frame.
a) copper alloys |
b) silicon steel |
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The commutator of a DC machine is made of copper bars mounted in
a) glass |
b) mica |
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The load current has no effect on the field excitation of a:
a. compound wound generator b. series wound generator c. shunt wound generator d. separately excited generator e. reverse wound exciter
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d. separately excited generator |
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Synchronous motors are used to
a) start alternators |
d) improve the power factor of a power system |
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Alternators are constructed to use slip rings made of brass or
a) copper |
e) steel |
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A power factor of less than _____ may result in an overcharge for the electrical users.
a. 0.5 b. 0.6 c. 0.7 d. 0.8 e. 0.9 |
d. 0.8
note: the system power factor should be kept close to 1. Poor power factors of 0.8 and less may be penalized for the distribution losses with increased power rates by the utility. |
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The field rheostat in a DC generator is used to adjust the _______________ of the motor.
a) direction of rotation |
c) output voltage |
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Apparent power is the power
a) used for the actual work |
c) supplied by the utility |
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Brush gear for DC machines have brushes made from carbon because they:
a. 2, 3, 5 |
c. 1, 2, 5 |
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Assigned numbers which designate a specific set of external dimensions for an electric motor refers to _______________.
a. frame number b. insulation c. duty d. service factor e. power |
a. frame number |
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A rheostat is also known as a
a) voltage regulator |
d) variable resistor |
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__________ are used on DC motors and generators to set up a magnetic field.
a. Insulated wires |
e. Field poles |
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The voltage induced in the secondary windings of the transformer is ________________ turns ratio of the transformer.
a) directly proportional to the |
a) directly proportional to the |
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Three major losses which occur in a transformer are iron losses, copper losses and
a) cooling losses |
c) flux leakage |
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Some of the hazards of transformer cooling oil can be compensated for by adding
a) chelates |
c) fire resistant liquids |
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A transformer has a primary voltage of 500 volts and 20 turns on the primary winding. If the secondary winding has 5 turns the voltage of the secondary is
a) 2000 volts |
c) 125 volts
500 volts / 20 turns = 25 volts-per-turn
25 volts/turn * 5 turns = 125V |
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Transformers are rated in terms of:
a) 1, 2 |
e) 1, 3 |
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If Ep / Es = Np / Ns then Ns will equal
a) Ep / Np Es |
e) Np Es / Ep |
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An active current transformer should never
a) be inspected until properly grounded |
d) have its secondary circuit opened |
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A device used to change the voltage and current from one level to another, is known as a/an
a) conductor |
d) transformer |
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Transformers should be inspected at least
a) quarterly |
d) annually |
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A transformer works on the principle of
a) reverse polarity |
b) magnetic induction |
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Current transformers step down the current rather than the
a) resistance |
e) voltage |
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An instrument transformer used to measure voltage is called a
a) current transformer |
c) potential transformer |
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Copper losses in a transformer are reduced by
a) using fan cooling |
c) using larger diameter wire |
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A phenomena produced by electronic switchgear which can result in excessively high voltages in transformers are known as
a) capacitive reactance |
e) harmonic distortion |
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Transformer cooling oil has
a) a very high specific heat |
a) a very high specific heat |
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When AC action causes a counter current in a coil it is termed
a) self inductance |
a) self inductance |
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A prime factor in determining the voltage to be used in a circuit is
a) transformer capacity |
c) degree of access to equipment |
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The best type of diagram for showing supply and distribution systems is the
a) riser diagram |
b) one-line diagram |
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When working with high voltage systems, the minimum distance of approach should be
a) 1 metre |
c) 2 metres |
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Low voltage systems may have an acceptable voltage variation of plus or minus
a) 10% |
b) 5% |
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The ability of an electrical circuit to store a charge of electricity even after the circuit is opened is termed
a) hysteresis |
e) capacitance |
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An electrical distribution system is composed of service, feeders and
a) secondary circuits |
c) branch circuits |
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The utility's responsibility will not go beyond the
a) first branch line |
b) first overcurrent device |
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The output voltage of a shunt dc generator may be varied by adjusting the |
a) field rheostat
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In an atom, the electrons: |
c) are attracted to the protons |
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The main components of a DC machine are the yoke or frame which supports the _______________ and the rotor which is called an armature. |
b) field poles
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Over correction of the power factor by capacitive reactance results in: |
d. 1,2,3
note: Capacitive reactance is an opposition to the change of voltage across an element |
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A power factor higher than ______ is generally not required as the savings do not equate to the cost of the extra capacitors. |
a. 0.95
note: A capacitor is a passive two-terminal electrical component used to store energy electrostatically in an electric field.
Unlike a resistor, an ideal capacitor does not dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field between its plates. |
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The three main types of DC generators are the: |
a) 1, 3, 5
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There are three main types of DC generators; series, shunt and compound wound. Each is |
d) self–excited
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Which of the following correction methods are used for power correction: |
e. 1,2,3
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Magnetic flux can be said to be |
b) lines of force outside a magnet and flowing N to S
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Most conductors tend to |
a) increase in resistance with increase in temperature
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_______________________ is an electromotive force. |
e) The volt
note: Electromotive force, (measured in volts), is the voltage developed by any source of electrical energy such as a battery |
